The present interpretation of the genetic organization of the cluster of loci comprising the mouse histocompatibility-2 (H-2) complex has been made possible through the examination of immunological traits in strains carrying genetic crossovers which took place within the H-2 complex. The research supported by this grant seeks to further the genetic definition of the H-2 complex by screening large numbers of backcross animals for the detection of unique crossover events with the subsequent development of inbred strains carrying the recombinant chromosome. Further, we seek to study the genetic organization of the S region of the H-2 complex by studying the expression of the Ss protein (fourth component of complement) in strains of mice carrying H-2 haplotypes derived from wild mice (designated B10.W strains). Both of these approaches have as their objective an improved understanding of the genetic organization of the H-2 complex.